[Amin_Mansouri]

Personally, I will suggest you to use SendKey.send method to send the “Enter” key to close ALTER automatically when the alert occurs. For more information and sample code, please check following links:
How can I close a javascript alert in WebBrowser: http://social.msdn.microsoft.com/Forums/...048d07cfbd

Kill Javascript "Alert Window" in Webbrowser Control: http://social.msdn.microsoft.com/forums/...c0e50f7bd/

If you not satisfied with this solution, you can find other solutions with this thread:

How to auto close webbrowser popup alert message in vb2008? Help: http://social.msdn.microsoft.com/forums/...57830C9752